Areas of Work
Groundwater

There are several isotopic methods and analyses of trace gases available to determine the age of groundwater. A competent estimation of age requires the inclusion of the boundary conditions like location and environment of the water resource, hydrogeological/geological cross sections, hydraulic characteristics, hydrochemical composition, supraregional studies and former investigations of the recharge area.

Depending on the age composition of the groundwater, environmental isotopes cover a wide range of time and scale from days to several millennia.

Very young groundwater recharge in the range of weeks, months or even a few years can be analysed over the compilation of time series of the stable isotopes oxygen-18 and deuterium.

For the determination of the mean residence time of young groundwater, recharged within the last 60 years, the radioactive isotopes tritium (³H) and krypton-85 (⁸⁵Kr) are very useful. By simultaneous determination, ³H and ⁸⁵Kr provide a possibility to quantify the age and fraction of the groundwater younger than 60 years.

The radioactive environmental isotopes (³H, ⁸⁵Kr, ¹⁴C, ³⁹Ar) mark surface waters on a global scale. Anthropogen activities are the main source for radioactive environmental isotopes besides cosmic radiation in the upper atmosphere. The greater part of ³H originates from nuclear weapon tests whereas ⁸⁵Kr is discharged from nuclear facilities. The input into the groundwater happens directly via precipitation (³H) or via dissolved soil gas in percolating water (⁸⁵Kr, ³⁹Ar, ¹⁴CO₂).

Due to longterm measurements at varous precipitation stations, the input of the environmental isotopes via the precipitation is well known. However, major local differences exist.

Radioactive environmental isotopes can also be used to estimate the age of old groundwaters in the range of hundreds to thousands of years. Primarily, the determination by carbon-14 (¹⁴C-DIC) in combination with carbon-13 (δ¹³C-DIC) has to be mentioned. By a simultaneous determination of ³⁹Ar and ¹⁴C, a contribution of mixtures can be recognised and quantified.

For the groundwater age range from thousand to ten thousand years, isotopes and gas parameters can be used as indirect analysis method indicating the climatic conditions of the recharge. This can be achieved by the determination of the stable isotopes oxygen-18 and deuterium or by measuring the noble gas temperature. This way, groundwater recharge during cold periods (Pleistocene) can be differentiated from recharge in warm periods (e.g. Holocene).

In case of groundwaters of high ages, the determination of the gas isotopes ³He/⁴He and ³⁶Ar/⁴⁰Ar are applied as indirect analytical methods.

Very high groundwater ages can be estimated by the activities of the radioactive isotopes chlorine-36 and krypton-81.

Groundwaters of different ages require a sophisticated handling in terms of their usage. The main characteristics of "old" groundwaters, mixed systems, and "young" groundwaters with regards to costs, efforts, protection, stability, and water abstractions are summarized in Tab.1.

Tab. 1: Characterisation of groundwater systems

Besides the groundwater age estimation several isotope methods are available to investigate evolution, recharge, and flow dynamics of groundwaters. All these methods use the isotope signature of dissolved compounds:

Isotope signatures of species of sulphur (sulphate, sulphide) provide information about the origin like evaporite solution, pyrite oxidation, fallout sulphur, etc., as well as secondary processes like sulphate reduction.

• Sulphur-34 and oxygen-18 of sulphate (δ³⁴S-SO₄ and δ¹⁸O-SO₄)
• Sulphur-34 of sulphide (δ³⁴S-H₂S)

Isotope signatures of species of nitrogen (nitrate, nitrite, ammonium, gaseous nitrogen) provide information about the origin such as organic or mineral fertiliser, geogenic origin, etc., as well as secondary processes like nitrate reduction.

• Nitrogen-15 and oxygen-18 (δ¹⁵N-NO₃ and δ¹⁸O-NO₃)
• Nitrogen-15 of ammonium (δ¹⁵N-NH₄)
• Nitrogen-15 of gaseous nitrogen (δ¹⁵N-N₂)

Isotope signatures of dissolved strontium provide information on the migration ways of groundwaters in different lithologies or over the fallout of nuclear accidents (e.g. Tschernobyl) respectively.

• Strontium isotope ratio (⁸⁷Sr/⁸⁶Sr)
• Strontium-90 (⁹⁰Sr)

Isotope signatures of anorganic and organic carbon components provide information about the origin such as volcanic gases, dissolved carbonates, organic materials, etc. and transformation processes like thermocatalytic, microbial, etc.

• Carbon-13 of DIC (δ¹³C-DIC)
• Carbon-13 and oxygen-18 of carbondioxid (δ¹³C-CO₂ and δ¹⁸O-CO₂)
• Carbon-13 of DOC (δ¹³C-DOC)
• Carbon-13 and deuterium of gaseous hydrocarbons (δ¹³C-CH₄ and δ²H-CH₄ as well as δ¹³C-C2-C4 and δ²H-C2-C4)
• Carbon-13 and deuterium of organic contaminations like HCH, BTEX, PAH

Isotope signatures of radioactive daughter products of the decay series of uranium-238, uranium-235, and thorium-232 provide information not only on the radioactive contamination of groundwaters but also on the origin, lithology, and changes in the flow dynamic.

• Activity concentrations of the radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra)
• Activity concentration of radon (²²²Rn)
• Activity concentrations of uranium isotopes (²³⁴U, ²³⁵U, ²³⁸U )
• Activity concentrations of radon daughter products (²¹⁰Pb, ²¹⁰Po)

Further isotope signatures of dissolved compounds like chloride, boron, lead, lithium, etc. are used for specific issues of geogenic origin or anthropogenic contamination (ammunition, mining, landfills, sewer, etc.).

• Chlorine-35 (δ³⁵Cl)
• Boron-11 (δ¹¹B)
• Lithium-6 (δ⁶Li)
• Lead isotopes (²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb)
• Iron
• Calcium
• Chromium

The input of radioactive nuclides from nuclear accidents and medical wastewater provide information not only on radioactive contamination but also on the age of soil samples.

• Caesium isotopes (¹³⁷Cs and ¹³⁴Cs)
• Iodine isotope (¹²⁹I and ¹³¹I)
• Americium

The study of the isotope signatures of δ¹⁸O and δ²H in groundwater can help answering several questions:

• Identification of the origin of waters
• Dating of young groundwaters
• Identification of glacial recharged groundwaters
• Infiltration of river water
• Infiltration of lake water
• Deuterium as artificial tracer for ground and pore water tracer tests as well as for biological applications

The hydrological usage of the content analyses of the stable isotopes δ²H- and δ¹⁸O- in water molecules is essentially based on the occurence of different concentrations in natural waters. Those differences are the result of several physical processes. Primarily they trace back to the evaporation depending on temperature. The various isotope effects cause a local and temporal characteristic marking of the precipitation and, consequently, also a local and temporal characteristic marking of the different water bodies of the hydrological cycle.

 Studying the stable isotopes should be included in every extended survey on groundwater age and origin. Those parameters are very suitable for all kind of monitoring programs to observe changes of the inflow.

The graph illustrates different groundwaters in a fictional region: